1. Field of the Invention
This invention relates to an electronic still camera. More particularly, it relates to an electronic still camera adapted for shooting or photographing a scene by using a solid-state imaging device to produce color video signals representing the scene.
2. Description of the Prior Art
The electronic still camera is designed, as its appellation implies, to produce color video signals for one field or frame captured on opening the shutter. Optimum exposure and white balance adjustment are required in order to produce a visually impeccable, reproduced image. In the electronic still camera, optimum exposure and white balance adjustment are always required for one-shot video signals. Optimum exposure means that the luminance signal or the green (G) signal obtained from the solid-state imaging device used for photographing the scene presents a substantially constant amplitude level without dependency on the light intensity of the scene. Optimum white balance adjustment implies making the signal level of the three colors of red (R), green (G) and blue (B) substantially equal to each other with respect to an image having no color tint.
Accordingly, the exposure control of the electronic still camera uses the same system as that for a conventional, silver-halide film still camera, for example, one in which the light transmitted through an objective lens is measured to adjust the shutter opening time as well as the light stop (aperture) value on the basis of a measure of the light intensity in order to give an optimum exposure to the solid-state imaging device. For white balance adjustment, a system is adopted in which the ratio of the R- to B-color components is determined by a color temperature sensor to adjust the amplification gains of the R-, G and B video signals on the basis of the determined ratio, similar to a system used in certain video cameras.
Video movie cameras employ an automatic iris control system in which the light stop value is controlled by the feedback control from the video signals for maintaining a constant video signal output level, or an automatic white balance adjustment in which the amplification gains are subjected to feedback control for providing a uniform output level of the R-, G- and B-signals. However, the feedback control system for these movie cameras cannot be used in electronic still cameras, since a faster response is required in the electronic still cameras in order to produce a one-shot scene or picture.
The solid-state imaging device used for electronic still cameras, such as the charge-coupled devices (CCDs), has a narrow dynamic range as compared to that of the conventional silver-halide photosensitive material type photographic film, so that it has not been possible to produce video signals representing an optimum image with the mutually irrelevant light exposure control by light measurement and white balance adjustment by detection of color temperature.
As the conventional white balance adjustment system, it is known to adjust the amplification gains of the R- and B-components of the color video signals produced by the solid-state imaging device so that the signal level of the integrated R- and B-components will be approximately equal to that of the integrated G-component. For example, the R- and B-signal components of the three color-separated signals R, G and B obtained from the solid-state imaging device are separately introduced into variable-gain amplifiers, and the mean signal levels of the output signals are monitored at the output sides thereof for determining the difference in the mean level, with the gains of two variable-gain amplifiers being controlled on the basis of the thus detected difference. This sequence of feedback operation is continued until the integrated signal levels of the R- and B-signals are about equal to each other. Thus it may be seen that, in this white balance adjustment system, the R- and B-signal levels are corrected with the G-signal as the reference. In other words, this system is predicated on the fact that the mean level of the G-signal remains constant without dependency on the color temperature.
However, in general, the signal level of the video signals developed from the solid-state imaging device depends on the color temperature of the incident light on the imaging device. For example, the output characteristics of the three color-separated signals R, G and B of the CCD solid-state imaging device are shown in FIG. 5, from which it is seen that the output level of the G-signal is lower at color temperatures below 3000.degree. K. than at color temperatures higher than 3000.degree. K. This chart shows changes in the output signal from the imaging device for the constant value of the integrated light intensity of the incident light on the solid-state imaging device. Hence, even if the amplification gains should be adjusted in dependence upon the color temperature of the scene as sensed by the color temperature sensor so that the mean signal level of the R- and B-signals are about equal to each other, an error may be caused to the mean signal level of the G-signal obtained at that time depending on the color temperature of the incident light. In such a case, an optimum white balance is not realized in the video signals produced from the gain-adjusted amplifiers.
In addition, the solid-state imaging device and the photodiode used for light measurement have different color temperature dependent sensitivities to the G-signal or luminance signal. For this reason, the luminance signal or the G-signal obtained from the solid-state imaging device cannot be made constant when the light transmitted through the object lens is sensed by a photodiode to control the shutter and/or diaphragm accordingly for adjusting the light exposure.
FIG. 6 shows for example the difference in the CCD sensor output with the color temperature in the case where the output of an auto-iris sensor of the silicon photodiode for light measurement is made constant. It may be seen from this figure that the luminance or brightness signal output Y obtained by a predetermined formula of signal mixing such as Y=0.3R+0.59G+0.11B, indicated by a chain line, is also changed with the color temperature, such that, in the vicinity of the color temperature of 3000.degree. K. becomes low since the sensitivity of the CCD sensor to the luminance signal is low.
Hence, in the system in which the shutter speed and/or the light stop value are adjusted in dependence upon the light intensity obtained by a light measuring device, such as a photodiode, the output G- or luminance signal level from the CCD imaging device cannot be adjusted to a constant optimum value in dependence upon the deviation in the spectral component of the incident light from the CCD imaging device, so that it is sometimes impossible to produce the video signals indicating an optimum image.
For example, the CCD imaging device is low in sensitivity at the color temperature of approximately 3000.degree. K., such that the output of the G-signal or the luminance signal from the CCD imaging device is low and is not constant even when the shutter speed and/or the light stop value is adjusted so that the G-component or the luminance signal is maintained at a predetermined level in accordance with the light intensity produced by the light measuring element.
In such a manner, in the conventional electronic still cameras, the light measuring device and the solid-state imaging device have different sensitivity characteristics to the color temperature, such that it is difficult to control the light exposure to provide constant output G- or luminance signals.
As noted hereinabove, the sensitivity of the solid-state imaging device to the G-signal is not constant and differs from that of the light-measuring photodiode output in dependence upon the color temperature. It is herefore necessary to adjust the G-component output so as to be constant in consideration of the color temperature and to correct the levels of the R- and B-signals using as the reference the G-signal made to be constant. However, when for example the light exposure is controlled so as to make the G-signal constant, the color temperature dependent sensitivities of the R- and B-signals are also changed by the changes in the light exposure. This results in changes in the amount of correction of the R- and B-signals aimed to adjust the white balance so that an optimum white balance has not been attained.